1. Field of the Invention
The present invention relates to a microdevice for measuring the electromagnetic characteristics of a medium and the use of said device.
It is used in the field of measuring electromagnetic characteristics of a medium to be checked. An example of such an application is the measurement of characteristics, particularly the conductivity and susceptibility, of geological deposits traversed by a borehole.
2. Brief Description of Related Prior Art
The term "logging" is used to define any continuous measurement of variations, as a function of the depth, of a given characteristic of deposits traversed by a borehole. The first logging tools were introduced by the Schlumberger brothers in 1927. The principle of the measurement consisted of measuring by means of electrodes the electrical resistivity of deposits traversed by boreholes filled with conductive muds. As a result of the ever more frequent use of non-conductive muds, for which the resistivity loggings by electrodes functioned poorly, Doll introduced in 1949 another resistivity logging principle, namely the measurement by electromagnetic induction (H. G. Doll, "Introduction to induction logging and application to logging of wells drilled with oil-based mud", Petroleum Transactions, pp. 148-162, 1949). Numerous devices operating according to one or other of the two aforementioned principles were designed and manufactured, making it possible to determine the resistivities of deposits with a vertical resolution from a few meters to a few dozen centimeters (the vertical resolution of a tool being the measurement of the finest band detectable by the tool).
However, the need to characterize deposits on finer scales for determining their dip has made it necessary to design very small devices, known as microdevices, which are mounted on blocks or pads bearing against the wall of the borehole. These microdevices aim at a good vertical resolution of approximately 1 cm. In the case of electric contact resistivity microdevices, such a resolution can be obtained as a result of the very small size of the electrodes. However, with the increasing use of non-conductive drilling muds, a need has appeared for induction-based conductivity measuring microdevices. The operating principle of such a device, such as is explained in an article by Wong Cho Chew et al entitled "Theory of Microinduction Measurements", (IEEE Transactions on Geoscience and remote Sensing, vol. 26, No. 6, November 1988) is shown in FIG. 1. A low frequency A.C. current I.sub.E =I.sub.o e.sup.i.omega.t flows in a so-called emitting or transmitting coil 1 producing a magnetic field of the same frequency, which in turn induces eddy currents I.sub.F in the deposit. These eddy currents are proportional to the conductivity of the rock and are phase-shifted by 90.degree. with respect to the emitting or transmitting current. In turn they induce in a so-called receiving coil 2 an electromotive force signal phase-shifted by 180.degree. with respect to the transmitting current. Frequently two receiving coils connected in opposition are used so as to eliminate the direct flux .phi..sub.1 generated by the transmitter and so as to only be sensitive to the secondary flux (.phi..sub.2) from the deposit. The resultant induced voltage is then directly proportional to the conductivity of the rock.
Despite several theoretical studies, such as that of Chew et al referred to hereinbefore, few practical solutions have been found to the resolution problem defined hereinbefore.
With such induction devices, it is only possible to obtain a vertical resolution of a few centimeters.
In addition, the practical limitations to the use of such devices are numerous, because they are sensitive to the actual drilling fluid, as well as to irregularities in the borehole wall, particularly to stand-off variations (sensor-wall spacing). These two phenomena can lead to interfering signals of greater magnitude than the signal of the deposit. Other interfering signals result from the coupling with other neighboring devices when several of them are located in the same borehole, or from the coupling of the device with the central support of the tool.